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Is one always reborn as a human being?

No, there are several realms in which one can be reborn. Some people are reborn in heaven, some are reborn in hell, some are reborn as hungry ghosts and so on. Heaven is not a place but a state of existence where one has a subtle body and where the mind experiences mainly pleasure. Some religions strive very hard to be reborn in a heavenly existence mistakenly believing it to be a permanent state. But it is not. Like all conditioned states, heaven is impermanent and when one’s life span there is finished, one could well be reborn again as a human. Hell, likewise, is not a place but a state of existence where one has a subtle body and where the mind experiences mainly anxiety and distress. Being a hungry ghost, again, is a state of existence where the body is subtle and where the mind is continually plagued by longing and dissatisfaction.
So heavenly beings experience mainly pleasure, hell beings and ghosts experience mainly pain and human beings experience usually a mixture of both. So the main difference between the human realm and other realms is the body type and the quality of experience.